1. Field of the Invention
The present invention relates to ultrasonic transducer for use in a fluid medium.
2. Description of Related Art
The present invention is based on known ultrasonic transducers that can be used for example in ultrasonic flow meters in process technology or in the automotive field, in particular in the intake and/or exhaust section of internal combustion engines, for volume flow or mass flow measurement. In this context, typically ultrasonic transducers are used that are capable both of emitting ultrasonic waves into a fluid medium (a gas and/or a liquid) and also receiving ultrasonic waves. Standardly, ultrasonic signals are transmitted through the flowing fluid medium from a transmitter to a receiver, and the runtime, runtime differences, or phases of the ultrasonic signals, or also combinations of these measurement quantities, are measured during this transmission. These signals are influenced by the flow of the fluid medium. From the degree of influencing of the runtime, it is possible to infer the flow speed of the fluid medium. An example of such an ultrasonic transducer that can be used for example in ultrasonic flow meters is presented in published German patent application document DE 10 2007 010 500 A1. The designs presented there can also be modified for use within the scope of the present invention. Using such ultrasonic transducers, for example air quantity signals can be derived within a system controlling of an internal combustion engine.
In standard ultrasonic transducers, as a rule piezoelectric transducer elements are used; however, such transducer elements have an acoustic impedance that differs greatly, for example by a factor of 6×105, from that of the surrounding fluid medium. Due to this large impedance difference, as a rule 99.9995% of the sonic energy on the path from the piezoelectric transducer element into the fluid medium is reflected back at the corresponding boundary surface, and cannot be used for the measurement. The same reflection loss occurs again at the second, receiving piezoelectric transducer element, which can also be identical to the first transducer element. In order to improve the acoustic coupling between the piezoelectric transducer element and the fluid medium, measures for impedance matching are standardly used. For example, from the prior art, e.g. from published German patent application document DE 10 2007 010 500 A1, ultrasonic transducers are known in which one or more matching bodies, in particular matching layers, for impedance matching are situated between the piezoelectric transducer element and the fluid medium. These matching bodies have acoustic impedances between that of the piezoelectric transducer element and that of the fluid medium. Thus, for example membranes or λ/4 layers onto which the (usually thin) piezoelement is glued can be used for impedance matching.
In known ultrasonic transducers, the connection between the matching body and the piezoelectric transducer element presents a particular technical challenge. In particular in the area of the connection, temperature shocks can cause damages that are due to different coefficients of thermal expansion (CTE). These coefficients indicate the relative change in length per change in temperature, in ppm/K or 10-6/K. For example, the coefficient of thermal expansion of many matching bodies is typically greater than 30 ppm/K, whereas most plastics and adhesives have a much higher coefficient of thermal expansion. In contrast, typical piezoceramics are in the range of 7 ppm/K. At the same time, however, piezoceramics generally react with extreme sensitivity to mechanical stresses, in particular tensile stresses and/or shear stresses, by forming microcracks or undergoing depolarization. Such stresses occur due to a rather slow relaxation of tension, in particular when there are rapid temperature shocks. Such a mechanical aging of the piezoceramics is usually significantly exacerbated by purely thermal or purely electrical loads that may be present.
In order to protect the piezoelectric transducer elements in the case of temperature shocks, for example flexibilized adhesives may be used. Such flexibilized adhesives, i.e. adhesives that are sufficiently flexible in themselves or are made sufficiently flexible by adding corresponding additives and/or filler materials, can compensate the above-described stresses due to different expansion of the matching bodies and of the piezoceramics. In this way, a sufficient stability under temperature shock of the ultrasonic transducers can be guaranteed. However, as a rule such flexibilized adhesives become so flexible at higher temperatures that they are no longer able to provide an adequate acoustic coupling between the piezoelectric transducer element and the matching body. Less flexible adhesives, in contrast, result in a larger usable temperature range of the ultrasonic transducer, but make the transducer more sensitive to temperature shocks. Thus, with regard to the coupling between the matching body and the piezoelectric transducer element there is a conflict between the goal of good acoustic coupling over a wide temperature range and the goal of high stability against temperature shocks.